The present invention relates to a production process and apparatus for economical and efficient production of high purity silicon, for example, Si with a purity of 99.999% or higher. Si of such purity can be utilized as Si for solar batteries.
Solidifying purification may be mentioned as a general process for metal purification. In the case of Si, heat-melted Si is cooled to solidification during which time the impurity elements other than Si are concentrated and condensed in the last solidified portion, and this portion is removed to obtain the purified Si. This process is based on the fact that most impurity elements have a small segregation coefficient for Si, but since the segregation coefficient of boron or phosphorus is close to 1 they are therefore difficult to remove by this process; in practice, therefore, it is difficult to achieve a purification of 99.999% or higher from Si with a purity of about 99% that is easily obtained by solidifying purification alone.
A common production process for high purity Si is a process widely employed in industry known as the Siemens process, whereby high purity Si is obtained by utilizing chlorides of Si; however, it is mainly suited for semiconductor related purposes, and while the purity achieved is above 99.999999% which is much better than the 99,999% required for solar battery Si, the production cost is high rendering it unsuitable for solar battery production.
One example of an attempt at easier production of Si at a purity that can be utilized in solar batteries is the process disclosed in Japanese Unexamined Patent Publication SHO No. 63-79717, whereby SiO gas is emitted from silica stone and metallic silicon and the gas is reduced by carbon kept at 1600 to 2400xc2x0 C. Another process found in U.S. Pat. No. 875,285 also reduces SiO with carbon, but neither of these processes deal with purification and no process is described for obtaining high purity Si. Reduction by carbon generally leaves carbon residue in the resulting Si since carbon is a solid, and it is therefore difficult to obtain high purity Si.
As a method of overcoming this problem, it has been considered that reducing SiO with a gas such as high purity hydrogen could minimize inclusion of impurities from the reducing agent. Below the melting point of SiO at about 1730xc2x0 C., SiO can only take the form of a solid or a gas with a pressure below the saturation vapor pressure, and in general, reaction with a reducing agent for reduction of solid or vapor SiO to obtain Si does not proceed very readily. The following process has been proposed as a solution.
Japanese Unexamined Patent Publication SHO No. 62-123009 describes a process in which silicon tetrachloride, trichlorosilane, silane and a silicon alcoholate are heat decomposed or flame heat decomposed to produce a fine particle aggregate of silicon monoxide and/or silicon dioxide, and the fine particle aggregate is reduced in a reducing atmosphere at 200xc2x0 C. or above to produce silicon. The size of the fine particles forming the fine particle aggregate is given as 10-100 nm, but since such fine particles are very highly reactive, they were thought to be susceptible to reduction. High purity silicon tetrachloride, trichlorosilane, silane and silicon alcoholate products are industrially produced and the resulting SiO fine particle aggregates are expected to be of high purity, so that reduction thereof gives high purity Si. However, silicon tetrachloride, trichlorosilane, silane and silicon alcoholate are costly, and therefore the final resulting Si is also costly.
One problem common to reduction processes occurs when the impurities in SiO become concentrated in the Si produced. The Si obtained by reduction of SiO is under 64 wt % of the SiO, and since the impurities in the SiO are not removed by high temperature reducing atmospheres, the impurities become concentrated in the Si which is under 64 wt % of the SiO, so that the resulting Si has a lower purity than the original SiO.
In U.S. Pat. No. 3,010,797 there is described a process in which silicon and silica are reacted to obtain SiO vapor which is reduced by hydrogen, and particularly a process in which it is reduced by hydrogen that has permeated through palladium or the like, or a process in which it is reduced by hydrogen in the copresence of it platinum. This process uses SiO obtained by reaction of silicon and silica, and both the silicon and silica starting materials are inexpensive and readily obtainable so that there is no problem with starting material cost. However, the following problems must be dealt with.
The first problem in U.S. Pat. No. 3,010,797 is that a large amount of hydrogen is necessary for hydrogen reduction of the SiO vapor obtained from the silicon and silica. While 90.5% of the total amount of Si contained in the SiO was obtained according to Example 1 of U.S. Pat. No. 3,010,797, the hydrogen required for this was 6 times the stoichiometric amount. When palladium is used in Example 3 of U.S. Pat. No. 3,010,797, hydrogen is required at 20 times the stoichiometric amount in order to obtain 86.5% of the total Si in the SiO. Since one mole of Si is about 28 g and one mole of hydrogen is about 22.4 L at room temperatures atmosphere, even if 100% of the Si in the SiO were obtained it would require 134-448 L of hydrogen to obtain about 28 g of Si by the reaction in this example. This is also clear from claim 1 of U.S. Pat. No. 3,010,797, where it is stated that an excess of hydrogen over the stoichiometric amount is necessary for reduction. This can be expected since most SiO is highly stable against reduction reaction, whether in gaseous or solid form. Except for specially synthesized SiO fine particle aggregates such as described in Japanese Unexamined Patent Publication SHO No. 62-123009 cited above, most SiO is stable and requires an excess of hydrogen for reduction by hydrogen to produce Si. From an industrial standpoint, a process that requires a few hundred liters of hydrogen to obtain 28 g of Si makes it difficult to achieve inexpensive production of Si, and hence improvement is desired in this aspect.
The second problem in U.S. Pat. No. 3,010,797 is that palladium or platinum is used for hydrogen reduction of SiO, as stated in claim 1 and throughout the specification. Use of these precious metals necessitates a more expensive reaction apparatus, while it is impossible to negate the risk of contamination, by these precious metals, of the resulting Si.
As described above, with processes for obtaining Si by reduction of SiO it is difficult to obtain high purity Si due to the inclusion of solid carbon when carbon is used as the reducing agent. Also, when a reducing gas is used as the reducing agent the reduction proceeds slowly and an excess of reducing gas is necessary. Another problem has been the required use of special costly SiO such as fine particle aggregates. Furthermore, the problem common to reduction processes is that the impurities contained in the SiO concentrate in the resulting Si, so that the resulting Si has a lower purity than the original SiO.
As another prior art finding that should be mentioned, U.S. Pat. No. 3,660,298 teaches that SiO vapor causes the disproportionation reaction: 2SiOxe2x86x92Si+SiO2 at about 1800xc2x0 C. According to the present invention there is provided, as will be explained below, a process whereby SiO2 solid is produced with Si, and impurities are concentrated in the SiO2 solid to increase the purity of the Si produced. However, the SiO2 by-product at 1800xc2x0 C. is liquid, and impurities will not concentrate in liquid SiO2 so that the purity of the Si produced cannot be increased. According to the process of the present invention it is possible to reduce the boron, etc. that cannot be removed by the aforementioned solidifying purification, and this is highly useful.
Moreover, while the present invention allows higher purification even in the process of obtaining Si from SiO solid, as just explained, production of high purity Si is favored when the SiO of the previous step is of as high a purity as possible. However, production of high purity SiO is not easy, as the conventional SiO production processes are associated with the following problems,
In U.S. Pat. No. 3,010,797 the high purification process is a process in which the higher vapor pressure of SiO compared to most other impurities is utilized for concentration of the SiO. However, there in fact exist impurities with a higher vapor pressure than SiO, and these impurities are impossible to remove. For example, phosphorus is an impurity that must be thoroughly removed, but phosphorus and phosphorus oxides have a vapor pressure which is the same or higher than SiO, and there is absolutely no removal effect by the method disclosed in U.S. Pat. No. 3,010,797, whereby instead of being removed the impurity concentration increases. Also, although the concentration of impurities with a lower vapor pressure than SiO is reduced, they evaporate in a portion corresponding to the vapor pressure so that it is impossible to completely eliminate them from the SiO solid. In other words, the low vapor pressure impurities are generally deposited at a higher temperature than SiO, and removal of these high temperature deposits has not been considered in the prior art.
As concerns these problems associated with high purification of SiO, the same may be said for the common SiO production techniques disclosed elsewhere besides U.S. Pat. No. 3,010,797. They are generally production processes in which silicon and silica are reacted at high temperature under reduced pressure and the resulting SiO vapor is condensed to solid; these simply utilize the high vapor pressure of SiO for high purification. These processes include the process whereby silicon and silica are heated after dry grinding to fine powder to obtain SiO, disclosed in Japanese Unexamined Patent Publication SHO No. 49-98807; the process utilizing quenching by adiabatic expansion of SiO vapor, disclosed in Japanese Unexamined Patent Publication SHO No. 59-8613; and the processes utilizing concentration in the vapor phase, disclosed in Japanese Unexamined Patent Publication SHO No. 62-27318, No. 63-103814 and No. 63-103815; in regard to high purification, however, all of these processes fail to provide an improvement in principle over the high purity SiO production in the first half of the process described in U.S. Pat. No. 3,010,797. That is, for purification they merely utilize the high vapor pressure of SiO, and impurities with higher vapor pressure such as phosphorus and phosphorus oxides are instead concentrated. Also, while some of the low vapor pressure impurities evaporate, the evaporated impurities are all included in the SiO solid.
Japanese Unexamined Patent Publication HEI No. 5-171412 describes a process whereby SiO is recovered on a recovery plate held at 1000xc2x0 C. to produce a low-splash SiO vapor deposition material. While Japanese Unexamined Patent Publication HEI No. 5-171412 does not deal with the purity, there is a problem in that the impurities that are deposited at a temperature higher than the temperature of the recovery plate are also deposited with the SiO solid, such that these impurities cannot be removed. Similarly, Japanese Examined Patent Publication SHO No. 40-22050 describes a process whereby SiO vapor is condensed in a vapor deposition tube kept at 450-950xc2x0 C. to facilitate stripping but, as in the case of Japanese Unexamined Patent Publication HEI No. 5-171412, the impurities that are deposited at a temperature higher than the temperature of the vapor deposition are also deposited with the SiO solid and therefore cannot be removed.
As explained above, these conventional processes have not allowed complete removal of impurities such as phosphorus and phosphorus oxides that have a high vapor pressure and are deposited at a lower temperature than SiO solid or of impurities that have a relatively low vapor pressure and are deposited at a higher temperature than SiO solid.
It is a first object of the present invention to provide a process for producing high purity Si economically and efficiently, by a process other than reduction from cheaply produced SiO solid, and a process for obtaining Si with higher purity than SiO solid, by high purification of Si in the course of producing Si from SiO solid. In particular, this allows an amount reduction of boron which has been impossible to remove by solidifying purification with common metal purification processes.
It is a second object of the invention to provide a process for achieving high purification in the course of producing SiO solid, and particularly to provide a process for removing impurities with a high vapor pressure such as phosphorus and phosphorus oxides that cannot be removed by the prior art processes, and for achieving even higher purification than the prior art even for low vapor pressure impurities by utilizing the difference in the deposition temperatures.
The invention achieves these objects by providing (1) a production process for high purity Si wherein solid silicon monoxide SiO is heated at a temperature of at least 1000xc2x0 C. and below 1730xc2x0 C., for a disproportionation reaction in which the solid SiO is decomposed to liquid or solid silicon Si and solid silicon dioxide SiO2, and the produced Si is separated from the SiO2 and/or SiO.
The following are also provided as preferred embodiments of the invention.
(2) A production process for high purity Si wherein the SiO solid is heated at a temperature of at least the melting point of Si and below 1730xc2x0 C.
(3) A production process for high purity Si wherein the SiO solid is heated at a temperature of at least the melting point of Si and below 1730xc2x0 C. and the produced liquid Si is separated, in liquid form, from the solid SiO and/or SiO2.
(4) A production process for high purity Si wherein the solid SiO is kept heated at a temperature of at least 1000xc2x0 C. and below the melting point of Si, and is then heated at a temperature of at least the melting point of Si and below 1730xc2x0 C.
(5) A production process for high purity Si, wherein the reaction is carried out under conditions such that, in terms of the molar ratio of Si and SiO2 produced by the disproportionation reaction represented by 1:x, x satisfies the inequality 1.5xe2x89xa7xxe2x89xa70.5.
(6) A production process for high purity Si, wherein the reaction system is substantially closed during the disproportionation reaction so that fresh atmosphere gas is not supplied to the reaction system, to prevent vaporization of the solid SiO.
(7) A production process for high purity Si wherein the flow rate of the atmosphere gas fed to the reaction system during the disproportionation reaction is controlled to realize said molar ratio of Si to SiO2 of 1:x (1.5xe2x89xa7xxe2x89xa70.5).
(8) A production process for high purity Si wherein the atmosphere gas supplied to the reaction system includes an oxidizing gas, and the flow rate of the oxidizing gas is controlled such that the molar ratio of Si to SiO2 of 1:x satisfies the inequality 1.5xe2x89xa7xxe2x89xa71.0.
(9) A production process for high purity Si wherein the atmosphere gas supplied to the reaction system includes a reducing gas, and the flow rate of the reducing gas is controlled such that the molar ratio of Si to SiO2 of 1:x satisfies the inequality 1.0xe2x89xa7xxe2x89xa70.5.
(10) A production process for high purity Si wherein the solid SiO is in the form of particles with a mean particle size of 1 xcexcm-5 mm.
(11) A production process for high purity Si wherein the impurity concentration of the produced Si is no more than {fraction (1/10)} of the impurity concentration of the solid SiO.
(12) A production process for high purity Si wherein the impurity concentrations of phosphorus and boron in the produced Si are lower than the impurity concentrations of phosphorus and boron in the solid SiO.
(13) A production process for high purity Si wherein the Si which is obtained is subjected to further high purification by a solidifying purification process.
(14) A production process for high purity Si wherein the SiO solid is obtained by a process whereby a starting mixture of carbon C, silicon Si or ferrosilicon, or a combination thereof with SiO2, is heated to generate SiO gas-containing gas, and the SiO-containing gas is cooled to produce solid SiO.
(15) A production process for high purity Si wherein during cooling of the SiO-containing gas, in a first zone of a first temperature which is at or above the deposition temperature of solid Si the solid impurities that condense and are deposited at the first temperature are first removed, solid SiO is then deposited in a second zone of a second temperature which is within the range of no higher than the deposition temperature of solid SiO and at least 300xc2x0 C., and the SiO deposited in the second zone is used as the solid SiO starting material for the disproportionation reaction.
(16) A production process for high purity Si wherein the cooling of the SiO-containing gas is carried out by introducing the SiO-containing gas at the high temperature end of a reaction apparatus with a temperature gradient from high temperature to low temperature and transporting it to the low temperature end thereof.
(17) A production process for high purity Si wherein the amount of impurities in the produced solid SiO is not more than {fraction (1/1000)} compared to the amount of impurities other than carbon C, silicon Si or ferrosilicon, or combinations thereof, and Si and O in the SiO2, in the starting mixture.
(18) A production process for high purity Si wherein the amount of phosphorus P in the carbon C, silicon Si or ferrosilicon, or combination thereof and the amount of phosphorus P in the SiO2, in the starting mixture, are both 1 ppm or less.
(19) A production process for high purity Si wherein the SiO-containing gas is rapidly cooled to produce the solid SiO in particle form.
(20) A production apparatus for high purity Si which comprises a thermal reaction section that houses and heats the starting materials to generate a SiO gas-containing gas, a SiO-containing gas cooling section that cools the SiO gas-containing gas produced in the thermal reaction section to condense and deposit SiO, and a thermal decomposition section that houses and heats the SiO solid produced in the SiO gas-containing gas cooling section to disproportionately decompose it to Si and SiO2.
(21) A production apparatus for high purity Si wherein the SiO gas-containing gas cooling section has a temperature gradient.
(22) A production apparatus for high purity Si wherein the SiO gas-containing gas cooling section has a SiO solid depositing region where SiO is deposited from the SiO gas-containing gas, and an impurity removing region between the thermal reaction section and the SiO solid depositing region, where the impurities that condense and deposit at a temperature above the deposition temperature of SiO are condensed and deposited from the SiO gas-containing gas and removed.
(23) A production apparatus for high purity Si wherein the SiO gas-containing gas cooling section is kept at 300xc2x0 C. or above in the region where SiO is condensed and deposited, and is provided with an exhaust mechanism for exhaust of the gas remaining after the SiO solid has been deposited.
(24) A production apparatus for high purity Si wherein the SiO gas-containing gas cooling section has a mechanism that cools the SiO gas-containing gas by adiabatic expansion or has a mechanism that cools the SiO gas-containing gas by introduction of a cooling gas thereinto, by which SiO solid in powder deposits form from the SiO gas-containing gas, and also has powder recovery means that recovers said powdered SiO solid.
(25) A production apparatus for high purity Si, wherein the thermal decomposition section has an extraction outlet at the lower end of a heat treatment vessel in which the SiO solid is heated, in order to draw out the molten Si.
(26) A production apparatus for high purity Si, wherein the thermal decomposition section has a SiO solid heat treatment vessel and a plurality of heating sections with independently settable heating temperatures, and also has a transport mechanism that transports the SiO solid between the heating sections.
(27) A production apparatus for high purity Si, wherein the heat treatment vessel has a plurality of zones whose heating temperatures are independently set by the plurality of heating sections and has a mechanism that transports the SiO solid between these zones.
(28) A production apparatus for high purity Si, which has a mechanism that transports the heat treatment vessel between the plurality of heating sections.
(29) A production apparatus for high purity Si, wherein a plurality of the heat treatment vessels are used, and there is provided a mechanism that transports the SiO solid from the heat treatment vessel situated in one of the plurality of heating sections to the heat treatment vessel situated in another heating section of the plurality of heating sections.